Mission-Critical MTC Devices
Mission-critical Machine Type Communication (MTC), C-MTC, devices used for communication in e.g. manufacturing, process industry, automotive or medical applications call for higher reliability and lower latency than previously supported in legacy systems (communication standards (generations, G) of the Third Generation Partnership Project (3GPP) 2G, 3G, 4G up to standard Release (Rel) 12) in order to maintain robust control loop functions. Message delays over the wireless link as well as the roundtrip time (RTT) must be kept low. Typical requirements are maximum message delay of no more than 20 milliseconds (ms) and only 0.001 parts per million (ppm) of messages violating this latency requirement. In order to meet such requirements in 4G, e.g. Long Term Evolution (LTE), strict requirements are put on the physical layer not to introduce transport block errors since each retransmission add 8 ms to the message delay. Moreover, interruption of the wireless link must be minimized.
Quality of Service
A standardized QoS concept is applied in 4G and has been designed to allow a service application (app) on a radio device such as a User Equipment (UE) to request end-to-end quality of service. Requirements to be fulfilled by the QoS mechanism are described in (3GPP Technical Specification (TS) 23.207 Version (V) 12.0.0 clause 4.1). It is explicitly stated that the QoS mechanism shall avoid unnecessary processing or complexity both in the UE and in the network nodes, and that unnecessary signalling traffic arising from QoS negotiations shall be avoided.
Each Evolved Packet System (EPS) bearer is associated with a QoS class indicator (QCI) which determines which scheduling policy, queue management policy, rate shaping policy, Radio Link Control (RLC) configuration etc. shall apply for data packets transported between the UE and the serving gateway (SGW) over that bearer (3GPP TS 23.401 V12.7.0 clause 4.7.2). In case of different applications having different QCIs those are mapped to different EPS bearers. Two types of bearers exist, those with guaranteed minimum bitrate (GBR type), and those without such guaranteed bitrate (non-GBR type).
User plane (data) bearers are activated, modified and deactivated by the UE or by the Packet Data Network, PDN, gateway (PGW). On UE side it is handled by the non-access stratum (NAS) which connects the UE to the Mobility Management Entity (MME) that relays the request to the SGW, which in turn relays the request to the PGW (see FIG. 1 for the network architecture of LTE). FIG. 1 shows an LTE architecture with a Radio Access Network (RAN) comprising a plurality of evolved Node B (eNB) base stations each serving one or several cells, with a UE connected to one of the eNB over a radio interface. Each eNB is connected to an SGW for user plane signalling and to an MME for control plane signalling over interfaces named in the figure. The RAN is connected to a PDN such as the Internet via a PGW. The UE specifies the required QoS when activating an EPS bearer, and may also request a modification of an existing bearer—for instance requesting change of the guaranteed bitrate or the QCI by sending a QoS information element with one or more modified parameter values (3GPP TS 24.301 V12.7.0 clause 6.5.4). The PGW responds by rejecting the request, modifying an existing bearer to the desired QoS, or activating a new bearer with the desired QoS to replace the bearer the UE requested to modify.
Standardized QoS levels are shown a table in 3GPP TS 23.203 V12.7.0 clause 6.1.7 for various QCIs. Priority levels are such that the lower the number the higher the priority. The packet delay budget covers end-to-end latency, including radio link(s) and other parts of the network infrastructure. The packet error loss rate is however only for the radio link since no packet loss is expected in the core network (CN) except for losses or latencies due to congestion. The actual packet delay shall in general be lower than indicated in the table for each of the QCIs, particularly for the GBR type of resources, provided that the UE experiences a sufficiently good radio link quality.
In case the eNB cannot sustain the bitrate guaranteed for a bearer it has no other option than to deactivate the concerned bearer since there is no mechanism for re-negotiating QoS between the eNB and the PGW (3GPP TS 23.401 V12.7.0 clause 4.7.2).
FIG. 2 illustrates the nomenclature of different bearers in LTE. Between the in UE and a peer entity (e.g. a service provider, SP) there is an end-to-end connection which comprises an EPS bearer between the UE and the PGW and an external bearer. The EPS bearer comprises an evolved radio access bearer (E-EAB) between the UE and the SGW, and an S5/S8 bearer. The E-RAB in its turn comprises a radio bearer between the UE and the eNB, and an S1 bearer over the S1 interface.